1. Field of the Invention
The invention relates to a method of producing a rare-earth magnet that is an oriented magnet, by hot working.
2. Description of Related Art
A rare-earth magnet using a rare-earth element such as lanthanoid is also called a permanent magnet. The rare-earth magnet has been used for a drive motor of a hybrid car or an electric vehicle in addition to a hard disk and a motor that constitutes an MRI.
As an index of a magnetic performance of the rare-earth magnet, residual magnetization (a residual magnetic flux density) and a coercive force may be exemplified. With an increase in amount of heat generation due to reduction of the size of a motor or an increase in the current density of a motor, demand for heat resistance of the used rare-earth magnet is further increasing. Accordingly, maintaining the magnetic properties of the magnet when the magnet is used under high-temperature is important.
Here, an example of a method of producing the rate-earth magnet in related art will be schematically illustrated with reference to FIGS. 8A and 8B and FIGS. 9A and 9B. In addition, FIGS. 8A and 8B are diagrams illustrating hot working in related art. Here, FIG. 8A is a schematic perspective diagram of a sintered body before the hot working (hot plastic working), and FIG. 8B is a schematic perspective diagram of the rare-earth magnet after the hot working. FIGS. 9A and 9B are explanatory diagrams of hot working in the related art. FIG. 9A is a longitudinal sectional diagram illustrating a relationship between a friction force that acts on the sintered body and a plastic flow during hot working, and FIG. 9B is a diagram illustrating a strain distribution of the rare-earth magnet in a longitudinal section CS of the rare-earth magnet in the related art shown in FIG. 8B.
First, for example, a fine powder, which is obtained by rapid solidification of Nd—Fe—B-based molten metal, is subjected to pressure forming to produce a sintered body Z shown in FIG. 8A. Next, the sintered body Z is subjected to hot working to produce a rare-earth magnet X shown in FIG. 8B. In the method of producing the rare-earth magnet X in the related art; a pressure is applied to an upper surface Z3 and a lower surface Z4 during hot working for the sintered body Z to compress the sintered body Z in an upper-lower direction that is a pressing direction, thereby causing a plastic flow in a horizontal direction perpendicular to the pressing direction. As a result, plastic deformation occurs.
At this time, when right and left side surfaces Z2, Z1 of the sintered body Z are in an unconstrained state and front and rear side surfaces Z5, Z6 of the sintered body Z are in a constrained state, the plastic flow is caused in the sintered body Z from the center in the right-left direction, whereby the right and left side surfaces Z2, Z1 are deformed. At this time, an upper surface Z3 and a lower surface Z4 of the sintered body Z are constrained by punches that apply a pressure thereto. When the sintered body Z, in which the upper surface Z3 and the lower surface Z4 are set in a constrained state due to the pressure applied by the punches as described above, begins to deform in the right-left direction, a frictional force acts on the constrained upper surface Z3 and lower surface Z4.
As shown in FIG. 9A, the frictional force F, which acts on the upper surface Z3 and the lower surface Z4 of the sintered body Z, is largest at the central portion CP in the right-left direction in which the sintered body Z is deformed, and the frictional force F decreases toward the right and left side surfaces Z2, Z1 of the sintered body Z. The frictional force F acts to hinder the plastic flow PF of the sintered body Z in the right-left direction. Accordingly, the plastic flow PF is less likely to occur (i.e., the ease, with which the plastic flow PF occurs, decreases) toward the central portion CP from the right and left side surfaces Z2, Z1 of the sintered body Z.
In addition, an effect of the friction force F on the plastic flow PF decreases toward the center of the inside of the sintered body Z in the pressing direction, that is, toward an intermediate portion between the upper surface Z3 and the lower surface Z4 from the constrained upper surface Z3 and lower surface Z4 of the sintered body Z. Accordingly, the plastic flow PF is more likely to occur (i.e., the ease, with which the plastic flow PF occurs, increases) toward the center of the inside of the sintered body Z in the pressing direction from the constrained upper and lower surfaces Z3, Z4 of the sintered body Z.
Accordingly, as shown in FIGS. 8A and 8B, when a pressure is applied to the upper surface Z3 and the lower surface Z4 of the sintered body Z to perform compression in the upper-lower direction while the right and left side surfaces Z2, Z1 of the sintered body Z are in the unconstrained state, a difference in the plastic flow is caused in a section CS that is parallel to the right-left direction and to the pressing direction. As a result, as shown in FIG. 9B, a strain in the section CS of the rare-earth magnet X that is produced becomes non-uniform. A non-uniform strain distribution is a factor for deteriorating magnetic properties of the rare-earth magnet X that is produced. Accordingly, it is necessary to prevent occurrence of the non-uniform strain distribution during production of a rare-earth magnet by the hot working.
As an example of the hot working in a process of producing the rare-earth magnet, Japanese Patent Application Publication No. 4-134804 (JP 4-134804 A) discloses a technology in which a cast alloy of a magnet is placed in a capsule, and die forging is performed at a temperature equal to or higher than 500° C. and equal to or lower than 1100° C. to make the alloy be magnetically anisotropic. In JP 4-134804A, when performing the hot working for the capsule using a forging machine, multi-stage forging is performed by placing the capsule in two or more kinds of dies. Thus, even in a thin capsule, it is possible to apply a pressure like a hydrostatic pressure to the inside of the forged alloy while causing plastic deformation in the cast alloy as in free forging. Accordingly, it is possible to prevent the magnet from being broken.
In a case where side surfaces of the sintered body are not constrained by dies as in JP 4-134804 A, the frictional force is largest at the central portions in the upper and lower surfaces. In addition, the effect of the frictional force is small at the central portion between the upper and lower surfaces of the sintered body, as compared to the vicinity of the upper and lower surfaces of the sintered body, and thus a relatively free plastic flow occurs at the central portion between the upper and lower surfaces of the sintered body, as compared to the vicinity of the upper and lower surfaces of the sintered body.
As a result, a difference in a strain amount in a lateral direction and a pressing direction is caused in the sintered body due to a difference in material flowability, and thus a strain distribution of a magnet becomes non-uniform in a section of the sintered body, which is parallel to the pressing direction. As the degree of working for the sintered body (the compression rate of the sintered body) increases, a difference in the strain amount between the vicinity of a surface of the magnet and the inside of the magnet increases. As a result, for example, when strong working in which the compression rate of the sintered body is approximately 10% or higher is performed, the strain distribution in a sectional direction of the magnet becomes significantly non-uniform. The non-uniform strain distribution is a factor for decreasing residual magnetization of the magnet.
On the other hand, Japanese Patent Application Publication No. 2-250922 (JP 2-250922 A) discloses a technology in which a rare-earth alloy ingot is placed in a metal capsule, hot rolling is performed at a rolling temperature equal to or higher than 750° C. and equal to or lower than 1150° C. in a state in which the alloy ingot includes a liquid phase, and hot rolling is performed in two or more passes so that a total working rate is 30% or higher. In JP 2-250922 A, rolling is performed while applying constraint from both sides of the metal capsule in a width direction. Thus, spreading in the width direction is suppressed during rolling of the alloy ingot. Accordingly, it is possible to obtain an appropriate crystal axis orientation in a width direction and a longitudinal direction of a long plate material that is obtained by the rolling.
However, in JP 2-250922 A, the metal capsule is not constrained in a longitudinal direction, and thus, almost all of a volume reduction due to a reduction of the metal ingot results in spreading in the longitudinal direction. Therefore, in a case where a plate material obtained by the rolling is a plate material having a predetermined length, and the plate material is not a continuous band plate, there is a possibility that the non-uniform strain distribution as described above may occur in a section along the longitudinal direction of the plate material. As described above, in the technologies disclosed in JP 4-134804 A and JP 2-250922 A, it may not be possible to prevent occurrence of the non-uniform strain distribution when the rare-earth magnet is produced through the hot working.